Which invention relates to the manipulation of solid, semi-solid or liquid materials in liquid media.
A wide variety of commercial processes involve the use of liquid media having solid, semi-solid or liquid particles suspended in them. The particles may vary very widely from inert inorganic materials through to reactive materials, and organic or biological structures such as cells or parts of cells.
It has been known for some time that particles of these various types may be caused to move within a liquid medium by the use of a non-uniform electric field, and the basic phenomenon of dielectrophoresis has been extensively discussed, for example see "Dielectrophoresis", Cambridge University Press, 1978 By H. A. Pohl and Chapter 6 of "Dielectric and Electronic Properties of Biological Materials", John Wiley & Sons 1979 by Ronald Pethig.
Recently the application of dielectrophoresis has been suggested in the area of materials classification: the construction of a so-called "optical dielectrophoresis spectrometer" is described in Burt, Al-Ameen & Pethig, "An optical dielectrophoresis spectrometer for low-frequency measurements on colloidal suspensions", Journal of Physics, Section E, Scientific Instrumentation, Volume 22 (1989) pages 952 to 957.
That paper, and the related paper "Applications of a New Optical Technique for Measuring the Dielectrophoretic Behaviour of Microorganisms", Price, Burt and Pethig Biochimica et Biophysica Acta 964 (1988) pages 221 to 230, disclose the use of interdigitated electrodes deposited on a dielectric substrate to cause movement of suspended particulates by the dielectrophoretic effect.
Most previous work has been directed to the characterization of a materials by taking appropriate measurements of their electric field-induced properties. Another major application is in the use of positive dielectrophoretic forces to align biological cells between electrodes prior to their electrofusion, as described by W. M. Arnold and U. Zimmermann ("Electric Field Induced Fusion and Rotation of Cells", Biological Membranes 5, 389-454, 1984). Also, a method and apparatus for dielectrophoretic manipulation of chemical species has been described by J. S. Batchelder (U.S. Pat. No. 4,390,403, Jun. 28, 1983). This method employs DC non-uniform electrical fields to manipulate one or more chemicals within a multi-electrode chamber so as to promote chemical reactions between the chemical species. The applied voltage may be periodically reversed in sign to decrease ionic shielding effects (see column 3, line 62 to column 4, line 3). The manipulation of the chemicals is controlled by positive dielectrophoretic forces resulting from differences in the dielectric constants of the chemical species.
In previous works of S. Masuda, M. Washizu and I. Kawabata "Movement of Blood Cells in Liquid by Nonuniform Travelling Field", IEEE Transactions on Industry Applications, Volume 24 (1988) pages 217 to 222, blood cells were caused to move under the influence of a non-uniform travelling electric field. This field was generated by applying two, fixed-frequency, multiphased, voltage signals, related by having the same frequency and amplitude, to a series of parallel electrodes. Likewise, the rotating electric field described by W. M. Arnold and U. Zimmermann and employed to cause rotation of a single cell, is generated using either a single, phase-split, voltage signal or synchronized, identical, voltage pulses.